Push-pull amplifier system



Oct. 13, 1949. g E 2,485,369

PUSH-PULL AMPLIFIER SYSTEM Filed Dec. 4, 1947 SOURCE Inventor: v RobertflDome,

His Attorney Patented Oct. 18, 1949 UNITED STATES PATENT OFFICEPUSH-PULL AMPLIFIER SYSTEM Robert B. Dome, Geddes Township, OnondagaCounty, N. Y., assignor to General Electric Company, a corporation ofNew York Application December 4, 1947, Serial No. 789,647

may be driven by a single-ended source. It is a primary object of myinvention toprovide improved means for obtaining push-pull output from asingle-ended source.

As it is well known in the art, it is desirable for high fidelity soundoutput to employ a pushpull out-put amplifier. In such systems as havebeen used in the past to obtain push-pull output from a single-endedsource, the output capabilities of the push-pull amplifier have not beenutilized to full advantage, since a series resistance has customarilybeen employed in the anode output circuit of one of thepair of outputtubes in order to obtain a driving voltage of suitable phase for thecontrol grid of the second output tube. Furthermore, some of the priorsystems proposed have an inherent tendency toward selfoscillation due toregeneration. In addition, some of these systems are characterized by acertain amount of undesirable hum which appears at the loud speaker.

It is a particular object of my invention to provide a push-pullamplifier circuit which obtains improved output. I

Another object of my invention is to provide such a system which is freeof any tendency toward self-oscillation between the push-pull tubes asis sometimes encountered because of the similarity of the circuits tothe Hartley oscillator. I

Another object of my invention is to provide such an amplifier in whichundesirable hum is substantially eliminated from the output.

A further object of my invention is to provide a push-pull amplifier,driven by a single-ended source, which employs a transformer in theanode circuit of the first-tube to supply grid excitation for the secondtube, thereby effecting an economy of parts. 1

The features of my invention which I believe to.

be novel are set forth with particularity in the appended claims. Myinvention itself, however, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in whichFig. 1 is a schematic circuit diagram of a push-pull amplifier systemwhich suitably embodiesmy invention, and Fig. 2 is'aschematic diagramofanother modification. Like numerals indicate like elements in the twofigures.

Referring to Fig. 1, an electron discharge device I having cathode,anode, and control electrodes 2, 3, 4, respectively, serves as asingleended audio amplifier. Audio-frequency signal voltage is appliedto the control electrode 4 of device I from a suitable source 5, whichmay comprise the second detector of a superheterodyne receiver. Thecustomary cathode bias resistor 6 and by-pass condenser l are providedbetween the cathode 2 of device I and the low potential side of theaudio-frequency source 5, which is shown connected to ground. Positivedirect operating voltage is applied to the anode 3 of device I from asuitable source, here shown as a battery 8, through the customary anodeload resistance 9. The amplified audio-frequency output appearing acrossthe load resistance 9 is coupled to the control lectrode H) of device H,which is one of a pair of electron discharge devices I I, [2 by means ofthe customary coupling capacitance l3 and rid leak resistance l4.Devices ll, I2 are connected in push-pull, the cathodes I5, I6 beingdirectly connected together and thence to ground through a commoncathode bias resistance I1 and by-pass condenser 18. The screen gridsI9, 20 of devices ll, l2 are directly connected together, positivedirect screen operating voltages being supplied from a suitable source,here shown as a battery 2|.

I provide an anode output circuit, comprising the parallel combinationof the primary winding 22 of a conventional output transformer 23 and acapacitance 24, connected in series between the anodes25, 26 of devicesII, [2 respectively. Also included in the anode circuit of device II isthe primary winding 21 of an auxiliary transformer 28, shunted by aresistance 29. Suitable positive unidirectional operating potentials aresupplied to the respective anodes 25, 26 from battery 2| through a tap30 on the primary winding 22 of output transformer 23. The secondarywinding 3i of output transformer 23 is connected to a suitable load,here shown as a loudspeaker 32.

In order to obtain true push-pull operation, the

secondary winding 33 of auxiliary transformer 28, being isolated fromthe primary winding 27 by an electrostatic shield 34, is connectedbetween the control electrode 35 of device 12 andground.

In operation, signals emanating from the audiofrequency source 5 areapplied to the control grid 4 of device I, and the amplified output isimpressed on the control electrode to of device I l' in a manner wellknown in the art. The resulting variations in the anode current ofdevice ll set up audio voltages across resistance 29 which are -degreesout of phase with the grid voltage.

A corresponding audio voltage is induced across secondary winding 33 ofauxiliary transformer 28 and applied to the control grid 35 of devicel2. Since the respective control grid voltages of devices II, I2 arealways opposite in phase, the varying anode currents of devices ll, [2combine in the primary winding 22 of the output transformer 2-3 toproduce additive effects on the load current in push-pull fashion. Theelectrostatic shield 34 between the windings of the auxiliarytransformer 28 is necessitated by the high audio-frequency potentialdiiference; be-. tween the windings.

It will be seen that the; use or am auxiliary transformer to supply gridexcitation; voltage for the second tube eliminates the necessity of thecustomary coupling condensen and. grid leak reasistor. A further economyof parts is effected by using an output transformer 23 havinga singietapped primary winding shunted by a single con- I denser,- instead ofemploying a circuit which usestwoprimary windingseach. shunted by acondenser. Furthermore, theuseof a separate phase inverter: tube is:avoided;

A furtheradvantage of the circuit of Fig. l is that any-hum: voltageappearing across the source of" the direct. operating potential:isisolated from the control grid: of the second: tube by. theauxilianytransformer; A still further advantage over a. system utilizing aresistance in. series. with. one half. of the primary winding; oftheoutputtransformer results from. the low direct voltage: drop in the:primary.- winding Ofi the auxiliary transformen. This; provides; an;increased output from a given. anode voltage: supply; Furthermore, thesystem; is inherently stable, ther e being very little tendency towardself-oscillation.

Referring to Fig. 2-, a. further. modification is shown in which theauxiliary transformer isv cons nected. in the low potential sideof; the:anodecathqde; circuit. In. this. application,v the parallel combination,comprising; resistance 29, and, the, primary winding, 21 of. the:auxiliary transformer 28: is connected. between the cathode; L of.device; M, and. the cathode. bias. resistance. I1. and by-passcondenser1.8,. In all. other. respects, the circuitsof. Fig, L and. Fig.2; areidentical. By, so, locating the auxiliary transformer, theinecessity of.an. electrostatic, shield between thewindings thereof; is.eliminated,sincelthe primary 2.1. is no, longer, at a high audiorfrequencypotential with. respect. to! the secondary. 31.. This, arrangement also.permits the. use, of. a. StSRr-HIJ. transformer; thus, by proper,desigmthe grid, excitation. vol'te age for device 1'2, may be madesubstantially the. samein magnitude as thatfor device. andatf the sametime the impedance of" the primary 2T may bemade low so astoavoidserious degeneration indevice l l. The use of step-up transformer.also... permits, the value, of resistance 29 to be decreased, thusproviding a still further increase. in. the net audio-frequency outputa1: pearing at the, loudspeaker 32 for agiven excitation voltage at.source 5. The magnitude. of the gridexcitation voltage applied to devicel2 may be controlled by suitably adjusting the value. ofiresi'stlance29'.

While Iihave shown and describedicertainpresent preferred" embodimentsof" my invention, it.

will" beund'erstocdi that numerous variations and;

modifications may be: made, and I contemplate inthe appended claims tocover any such variations and modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination with a single-ended source of signal voltage, anelectronic amplifier system comprising a pair of electron dischargedevices connected; in push-pull, said devices, having respective anodesand controlelectrodes, said source being connected to the controlelectrode of one of said devices, means comprising a first transformer.for coupling the output of said system to a useful load, a secondtransformer having a: primary winding connected in the anodecathodecircuit of said one device and having a resistanceirr paralleltherewith, and the secondany of' said second transformer being connectedbetween the control electrode of the other of said devices and ground,and means for applying substantially equal positive unidirecti'onaboperating potentiads; to) said. anodes;

22 In combinatiom with; a sing-lesended. source of: signal; voltage;v amelectronic: amplifier: system comprising; 3.1. pain. oft elect/mmdischarge.- devices connected irr nushz-rmlli, said devices having. re,-spective anodes and: control; electrodes, said source: bein connectedto. the-contnolelectrode of one; of saidi devices... a nusiepull.output; circuit for said: devices.- comprising; a transformer having, aprimar winding and a second y" Winding. id primary. winding; beingconnected in; series with the; anode; of said. one: dlwice; said.secondary winding, being conncctedsbetween. the-controlrelectrode; of.the other: of saiddevlices. andithe. low potential. side.- of. said;source, aresistance. connected in Harallehwith.saidlprimary. winding andmeans for applying, substantially eqnaL positive direct operatingpetentials, to. said anodes 3.11:1, combination. with a single-ended.source of. signal.-. voltage; an electronic amplifier system comprising,a pain of electron. discharge. devices connected. in. push-pull; said.devices having respective cathodes, anodes, and" contr01..ele.ct'rod'es,said. source. being connected. to the. control; electrode of one. ofsaid devices, an. output, circuit connectedbetween said" anodes, meanscomprising, a transformerhaving primary and secondary windings;for'applying signalvoltage to the control' electrode. of the: other" ofsaid devices in proper phase: for push-pull operation; said primary"winding being-i connected in series. between said cathodes and in'parallel" with a resistance; said secondary winding being connectedbetween the control electrode of said second device and ground; andmeans for applying substantially equal positive unidirectional"operating potentials tosaid anodes:

RQBEBFT. B. EIGJME;

REEEBENQESS GLDED The following references" are of record iir the fileof this patent:

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